Exposure system for polymeric materials

ABSTRACT

A method and apparatus for exposing photosensitive recording material is presented. The invention provides for using chromatic lenses to focus polychromatic light onto recording media. The resulting chromatic aberrations contribute to exposing recording media in three dimensions. The method and apparatus presented also provide for improved quantum efficiency in the exposure of photosensitive recording material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates in general to a method forrecording information oh optically sensitive media, and moreparticularly to a method suitable for recording information on proofingand lithographic printing plates.

[0003] 2. Description of Related Art

[0004] Traditional techniques of introducing an image onto recordingmedia include photoresist, color proofing, letterpress printing,flexographic printing, gravure printing and offset printing. Thesetraditional techniques use various light sources to exposephotosensitive materials with analogue lithographic masking filmtechnology. The quality of exposure contributes to the resolution,durability and overall condition of the image in the final products.

[0005] Digitally modulated lasers are typically used to expose recordingmedia. Lasers have been the preferred light source for digital imagingdevices because they provide collimated, monochromatic emission.Collimated single wavelength light beams are easy to modulate and do notsuffer from chromatic aberrations, thus eliminating the need forsophisticated achromatic optics. Collimated single wavelength light,however, has some drawbacks in the application of recording informationon optically sensitive media.

[0006] Most photosensitive recording materials form three-dimensionallayers with a discrete thickness. For example, in offset printing platesthe photosensitive materials are typically coated as layers 0.5-10micron thick on a substrate, and flexographic printing plates are coatedor extruded as a 1-7 mm thick layer on a substrate. Although therecording media is a three-dimensional layer, the monochromatic lightsources typically used to photo-expose the recording media generate asubstantially two-dimensional focal point, as schematically shown inFIG. 1. A laser light source 14 is focused through a chromatic lens 16onto a photosensitive layer 12. The monochromatic light is focused to atwo-dimensional focal point 18 at the surface of the photosensitivelayer 12. A two-dimensional single focal point optimally exposes onlyone level in the three-dimensional photosensitive layer. In practice,the two-dimensional focal point is typically projected at the surface ofthe layer of photosensitive material, and the photosensitive materialbelow the surface is not sufficiently photo-exposed.

[0007] There are many problems associated with insufficientphoto-exposure below the surface of the photosensitive layer.Insufficient photo-exposure results in inadequate initiation of thephotochemical reaction in the recording medium. The photochemicalreaction cures the recording medium and the reaction is the foundationfor all the subsequent processes in printing applications. The practicalimplications of insufficient photochemical reaction in the recordingmaterial include poor printing run length, poor ink receptivity, sideetching and poor bonding to the substrate.

[0008] Since photo-exposure of recording media is so critical inprinting applications, many adaptations have been made to optimize thephoto-exposure process. One adaptation is the use of short wavelengthlight sources. Short wavelength radiation has higher energy to initiatethe photochemical reaction in recording materials. Shorter wavelengthsof light also provide higher resolution capability than longerwavelengths. However, short wavelength radiation does not penetrate wellbelow the surface of the photosensitive material. Shorter wavelengths oflight reflect off the surface of recording media more than light oflonger wavelength. Higher reflectance off the surface leaves lessradiation available to photo-initiate the desired chemical reaction.

[0009] In addition to exhibiting a greater degree of reflection, shortwavelength light also refracts more than longer wavelengths. Greaterrefraction has two effects that contribute to poor photo-exposure ofrecording media. The shorter wavelengths lose more energy through lensinterference because they have a longer path through the lens materialthan the longer, less refracted wavelengths. Also, the highly refractedshorter wavelengths of light angle sharply from the focusing lens to thefocal point, so light penetration is very shallow. As a result, highlyrefracted light does not pass through photosensitive material below thesurface.

[0010] In addition to poor penetration below the surface of therecording material, lasers have other shortcomings as exposure sourcesin digital imaging. Photosensitive recording materials havepolychromatic photosensitivity. Many photosensitive recording media havebroad spectral sensitivity, and absorb monochromatic radiation withrelatively low quantum efficiency. The photochemical reaction may beinitiated with a range of wavelengths, and when only a single wavelengthis used, photo-initiation is very inefficient. In order to combat lowquantum efficiency, very high intensities are required. High intensityradiation can saturate single absorption bands, which causes failure ofthe reciprocity law. The practical consequences of absorption bandsaturation relevant to printing applications include halation, fog andpoor resolution in the printed image.

[0011] In order to avoid the difficulties with high intensity radiationan ideal light source would have spectral emission similar to thespectral sensitivity of the material. Such spectral overlap could beachieved with an appropriate polychromatic light source. Polychromaticlight sources have been used in combination with achromatic lenses toexpose photosensitive recording media. Achromatic lenses correct forchromatic aberrations, which are caused from the various wavelengths oflight from polychromatic sources. Achromatic optics combine opticalelements with low and high indices of refraction to correct forchromatic aberrations. Achromatic lenses focus light with no chromaticaberration so that all the wavelengths of polychromatic light focus atthe same focal point. Because achromatic lenses focus all wavelengths oflight to a two-dimensional focal point, this exposure method alsoprovides insufficient photo-initiation for the recording material belowthe surface of the layer.

[0012] An illumination method that facilitates polychromatic exposure ofa layer of photosensitive material from the surface to the substratewould alleviate many of the aforementioned problems in printingapplications.

SUMMARY OF THE INVENTION

[0013] Polychromatic light sources provide efficient energy in terms ofquantum absorption for photosensitive recording media, but achromaticlenses focus the polychromatic light to a single two-dimensional focalpoint, which prevents exposure of the photosensitive material below thesurface of the three-dimensional layer. The present invention takesadvantage of the spectral breadth and focal point distribution providedby polychromatic light sources. As a result of these attributes,polychromatic light sources have higher quantum efficiency and permeatedeeper below the surface of photosensitive recording media thanmonochromatic light sources.

[0014] Therefore, it is an object of the present invention to provide anillumination method by which polychromatic light intensity can bedistributed in a three-dimensional focal point onto recording media.

[0015] The above object is obtained by a method for exposing athree-dimensional layer of recording media with an imaging system. Theimaging system comprising a polychromatic light source that providesradiation to an optical modulator; the imaging system further includinga system of achromatic optics and a chromatic lens along with the layerof recording media. The method comprises modulating a light beam fromthe polychromatic light source with the optical modulator andcollimating the light beam from the optical modulator with the system ofachromatic optics. The method further comprises focusing the light beamfrom the system of achromatic optics with the chromatic lens andexposing the layer of recording media with the focused light beam fromthe chromatic lens.

[0016] The present invention also provides an imaging apparatus forexposing a three-dimensional layer of recording media. The imagingapparatus comprises a polychromatic light source, an optical modulator,a system of achromatic optics and a chromatic lens. The opticalmodulator modulates a light beam from the polychromatic light source andthe system of achromatic optics collimates the modulated light beam. Thechromatic lens focuses the light beam onto the layer of recording media.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention can be more fully understood from the followingdescription thereof in connection with the accompanying drawingsdescribed as follows.

[0018]FIG. 1 schematically illustrates a two-dimensional focal pointfrom a laser on a three-dimensional photosensitive layer.

[0019]FIG. 2 schematically illustrates the wavelength dependence oflight refraction, as shorter wavelengths have larger indexes ofrefraction than longer wavelengths of light.

[0020]FIG. 3 schematically illustrates a preferred embodiment of apolychromatic illumination system assembled according to the presentinvention.

[0021]FIG. 4 is a block diagram of an exposure system built inaccordance with the present invention.

[0022]FIG. 5 schematically illustrates the chromatic aberrationvariation between chromatic lenses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The method of illuminating photosensitive material offered by thepresent invention exploits the properties of polychromatic light informing chromatic aberrations when focused. As used herein, the term“polychromatic light” refers to light of more than one wavelength,typically a continuous spectrum of wavelengths of different energy.Accordingly, the term “monochromatic light” refers to light of a singlewavelength, such as light generated by laser sources. The term“chromatic lens” refers to an optical component of one or more opticalelements that separates polychromatic light in a prism-like manner, withshorter wavelengths focusing closer to the lens than longer wavelengths.Polychromatic light focused with a chromatic lens results in chromaticaberrations, the continuous distribution of focal points between theshortest and longest wavelengths. The term “achromatized optics” refersto an optical component of more than one optical element that focusesall the wavelengths of polychromatic light at the same distance from thefocusing lens. Polychromatic light focused with achromatized optics doesnot exhibit chromatic aberrations.

[0024] Throughout the following detailed description, similar referencecharacters refer to similar elements in all figures of the drawings.

[0025] Polychromatic light sources give rise to chromatic aberrations,which are caused by the differences in refraction of the variouswavelengths of the spectrum. Shorter wavelengths have greater indices ofrefraction than longer wavelengths and this causes wavelengths ofdifferent energy to separate upon refraction by a lens. The effect ofchromatic aberration is illustrated in FIG. 2. A polychromatic lightbeam 20 is focused by a chromatic lens 16′ and the various wavelengthsseparate and are focused at different focal points 22, 24, 26. Theshortest wavelengths have a focal point 22 closest to the lens. Thelongest wavelengths have a focal point 26 furthest from the focusinglens. The distribution of focal points between the longest and shortestwavelengths reflects the spectral distribution of the light source.

[0026] Referring to FIG. 3, the present invention provides a system toilluminate a three-dimensional photosensitive layer 12″ which isdeposited on a substrate 10″. As shown, in FIG. 3 there is apolychromatic light source 36 having an optical modulator 38 forrecording an image on the photosensitive medium 12″. The preferredpolychromatic light source is selected to compliment the spectralsensitivity of the recording media. Typically light sources withemissions in the UV/VIS region of the spectrum, such as the HI-IQ UVsystem manufactured by Fusion UV Systems Inc. are used. An acousticoptical modulator (AOM), an acoustic optical deflector (AOD) and fibercupping all together, represented as an optical modulator 38, modulatethe polychromatic light. The modulator 38 is chosen to correspond to thecharacteristics of the light source, modulation speed and the requiredsize of the writing spot. In one preferred embodiment of the invention,the modulator is a Model ASM series, designed for UV, manufactured byInterAction Corp. Other modulators may be selected according towavelength region and range specifications of the light source.

[0027] The illumination system illustrated in FIG. 3 shows the modulatedpolychromatic light beam 20′ passing through an imaging system ofachromatic optics 32 and a chromatic lens 28. The achromatic optics 32aligns the beam while correcting for chromatic aberrations. Achromaticlenses are standard in the art and are commercially available from manymanufacturers, such as Nikon, Hoya and Edmund Scientific Co. In onepreferred embodiment of the present invention the achromatic lenses weremade with a focal point distance of 100 mm, and were made with BK7-SF5glass.

[0028] Achromatized optics typically comprises the combination of a highindex or refraction lens and a low index of refraction lens. Sometypical material combinations for use in making achromatized lenses are;BaF10-FD10, BaFN10-SF57, SSKN8-FD10, SSKN8-SF10, F2-K5-F2, F2-BK7-F2, inaddition to other combinations.

[0029] Achromatic optics provides a light beam for the illuminationapplication that includes all wavelengths of the light source emissionin the same wavefront. Again referring to the system illustrated in FIG.3, a chromatic lens 28 is next used to focus the beam from theachromatic optics. Chromatic lenses are the simplest kind of focusinglens, low cost and readily available for multiple applications. In oneembodiment of the present invention a chromatic Plano-Convex (PCX)single lens made from BK7 glass with a focal point distance of 100 mm isused.

[0030] Chromatic lenses are not corrected for chromatic aberrations andthus the various wavelengths of light are focused at the variousrespective focal points 40, 42 and 44 as illustrated in FIG. 3. Thechromatic lens 28 is selected so that it focuses the light 34 such thatthe spectral emission of the light source is focused throughout thedepth of the photosensitive layer 12″.

[0031] The chromatic aberration effect of the chromatic lens 28 spreadsthe focal point of the light so that the light penetrates below thesurface of the photosensitive material layer 12″. The shorterwavelengths are focused closest to the lens 28, at the surface of thephotosensitive material 12″, as shown in FIG. 3. The focal points of theincreasingly longer wavelengths penetrate below the surface into thelayer of the photosensitive material. The longer wavelengths of lightphoto-initiate the recording material below the surface of the layer.

[0032] The longer wavelengths of light penetrate well below the surfaceof the recording material because of two complimentary phenomena; thelonger focal points due to the chromatic aberration effect discussedabove, as well as the reflection properties for longer wavelengths ascompared to shorter wavelengths of light. Radiation of longerwavelengths exhibits less reflectance off the surface of thephotosensitive layer than does shorter wavelengths. As the reflectancedecreases, more radiation penetrates below the surface of the layer.

[0033] In addition to the above mentioned optical phenomena that supportthe use of polychromatic light sources in exposing photosensitivematerial, the use of polychromatic light benefits from the quantumefficiency in the absorption phenomenon. Polychromatic light sourcesprovide a distribution of energy that can be optimized to substantiallyoverlap with the absorption profile of the photosensitive material. Anenergy distribution, in contrast with monochromatic radiation, utilizesa variety of potential absorbers in the photosensitive material. Sincequantum absorbers of various energies contribute to photo-activation,lower intensity of polychromatic light provides more extensive exposureas compared to higher intensity monochromatic light. Due to improvedquantum efficiency provided with exposure polychromatic light sources,poor image quality due to quantum saturation of the photosensitivematerial is avoided. The method and apparatus of the current inventionemploy the benefits of the aforementioned optical and physicalproperties in exposing photosensitive material.

[0034] The chromatic lens specification determines the focal pointpenetration below the surface of the layer of photosensitive material.The distance between focal points from a polychromatic light source is afunction of the chromatic lens index of refraction, as illustrated inFIG. 5. The thin lens 40 of FIG. 5 has a lower index of refraction thanthe thick lens 46. The distance S1 between the shortest focal length 42and the longest focal length 44 of lens 40 is much less than thedistance S2 provided by lens 46. The lengths S1 and S2 represent themaximum exposure length possible with their respective lenses. Exposurepenetration can be optimized through appropriate selection of the indexof refraction of the chromatic lens in accordance with the spectralprofile of the light source. In accordance with the present invention, achromatic lens may comprise two to three optical elements that somewhatcorrect for chromatic aberration, yet the focused light retains somechromatic aberration. The distance between the shortest focal length andthe longest focal length the chromatic aberration of a lens may also beoptimized using multiple optical components.

[0035] In operation, a photosensitive recording medium, which may be aflat surface or the surface of a drum having an outer photoconductivelayer, is scanned (by means not shown) in two dimensions. It will beevident that there are other recording media that could be used,including photoconductively coated belts and plates, as well asphotosensitive films and coated papers. Thus in the generalized case,the recording medium should be visualized as being a photosensitivemedium which is exposed by a light source.

[0036] Now referring to the block diagram in FIG. 4 where the presentinvention is illustrated as an imaging system for a variety ofphotosensitive imaging processes. In the preferred embodiment of thepresent invention, the system for imaging on recording media requires alight source 50 that provides a beam 51 of polychromatic radiation. Anoptical modulator 52 is incorporated into the system to control the beamintensity. The achromatic optics 54 adjusts the light beam from themodulator to form a chromatically corrected beam.

[0037] The next element of the imaging system of the present inventionis a chromatic lens 56. The chromatic lens 56 focuses the collimatedlight beam. Various embodiments of the present invention are possible.This imaging system can be used to expose recording media 58 directly,or in conjunction with any deflector system 57. The deflector system 57may include a raster scanning system such as a drum, a mirror, or anydeflector device in the illumination of photosensitive recording media58′.

[0038] Another embodiment of the present invention is a method forexposing photosensitive material. The method comprises providing a beamof polychromatic light to an optical modulator, modulating the beam andsubsequently focusing the polychromatic beam with achromatic optics andthus providing a wavefront of radiation free of chromatic aberrations toa chromatic lens. The methods further comprises focusing the radiationbeam with the chromatic lens onto photosensitive recording media, or adeflecting device which then directs the beam from the chromatic lens tophotosensitive recording media.

Those having the benefit of the above description of my invention mayprovide numerous such modifications of the invention. Thesemodifications are to be construed as being encompassed within the scopeof the present invention as set forth in the appended claims wherein Iclaim:
 1. A method for exposing a three-dimensional layer of recordingmedia with an imaging system comprising a polychromatic light sourcethat provides radiation to an optical modulator; the imaging systemfurther including a system of achromatic optics and a chromatic lensalong with said layer of recording media, the method comprising: (a)modulating a light beam from the polychromatic light source with theoptical modulator; (b) collimating the light beam from the opticalmodulator with the system of achromatic optics; (c) focusing the lightbeam from the system of achromatic optics with the chromatic lens; (d)exposing the layer of recording media with the focused light beam fromthe chromatic lens.
 2. The method according to claim 1 wherein the layerof recording media has a surface and the light beam from the chromaticlens has a focal point distribution that extends from the surface ofsaid layer of recording media to below the surface of the layer ofrecording media.
 3. The method according to claim 1 wherein the layer ofrecording media is a photosensitive material.
 4. The method according toclaim 1 wherein a deflector device directs light from the chromatic lensto the layer of recording media.
 5. The method according to claim 4wherein the deflector device is a mirror.
 6. An imaging apparatus forexposing a three-dimensional layer of recording media comprising: a) apolychromatic light source, b) an optical modulator, c) a system ofachromatic optics and d) a chromatic lens wherein the optical modulatormodulates a light beam from the polychromatic light source, the systemof achromatic optics collimates said modulated light beam, and whereinthe chromatic lens focuses the light beam onto the layer of recordingmedia.
 7. The apparatus according to claim 6 wherein the layer ofrecording media has a surface and the light beam from the chromatic lenshas a focal point distribution that extends from the surface of saidlayer of recording media to below the surface of the layer of recordingmedia.
 8. The apparatus according to claim 6 wherein the layer ofrecording media is photosensitive.
 9. The apparatus according to claim 6wherein a deflector device directs light from the chromatic lens to thelayer of recording media.
 10. The apparatus according to claim 9 whereinthe deflector device is a mirror.